1. Field of the Invention
The present invention relates to a process for producing sintered ore used as a raw material in ironmaking and sintered ore produced by means of the process and particularly, to a process for producing the sintered ore which product shows improved strength, and sintered ore produced by means of the process.
2. Description of the Prior Art
Production of sintered ore used as a raw material in ironmaking is generally performed in a preliminary process for a sintering raw material shown in FIG. 5. This preliminary process includes steps of: mixing and granulation with raw material tanks 1 and a drum mixer 2; and sintering with an ore feed hopper 3 and a sintering machine 4. The sintering raw material is composed of: iron ore powder of about 10 mm or less in particle size; auxiliary materials, such as lime stone, quick lime, silica rock and serpentine; and solid fuel, such as coke, and components of the sintering raw material are stored in the raw material tanks 1. The components of the sintering raw material are charged into the drum mixer 2 at a predetermined composition, then, a proper amount of water is added thereto and the components are mixed and granulated in the drum mixer 2. Granules thus formed are packed on a pallet of a sintering machine (for example, a Dwight-Lloyd sintering machine) 4 to a predetermined height by the ore feed hopper 3 and solid fuel in a top surface section of the packed raw material is ignited. After the ignition, burning of the solid fuel is continued while the air is sucked downward and the sintering raw material is sintered by a combustion heat to form a sinter cake. This sinter cake is pulverized, followed by particle size adjustment to obtain a sintered ore product of 3 mm or more in particle size.
A sintered ore product requires high strength as a raw material in ironmaking. This is to prevent not only reduction in production yield due to pulverization of sintered ore in the course of charging it into a blast furnace but deterioration in blast furnace operating conditions due to reduction in air flowing-through property through a blast furnace from pulverization of sintered ore in the furnace.
In order to improve the strength of the sintered ore product, it is important that a high temperature is generated in the granules packed on the pallet of the sintering machine by combustion of solid fuel in the granules and this high temperature is maintained, so that a sufficient amount of a melt for sintering of iron ore powder is formed in a uniform manner in the bulk of the sintering raw material.
The melt is formed by a slag reaction between iron ore and the auxiliary materials, the melt being, in general, a multi-component system calcium-ferrite. With the melt, liquid phase sintering of iron ore powder is effected and after cooling, bonds between particles of the iron ore powder are formed.
It has widely been known that the strength of a sintered ore product is increased either when a bond is wide or when bonds are constructed in a network structure.
For this reason, it has been understood that the strength of a sintered ore product can be improved when a sufficient amount of a melt for sintering iron ore powder is produced to extend a width of a bond and in addition, the melt is produced in a uniform manner in the bulk of the sintering raw material to achieve a uniform network structure of bond.
An air flowing-through resistance of a granule bed packed on the pallet of the sintering machine has generally been reduced in order that a high temperature is generated and maintained by combustion of solid fuel included in the granules as described above. With the reduced air flowing-through resistance, much of air can be passed through the bed and the solid fuel can be efficiently burned in a uniform manner, thereby enabling a high temperature at which a high strength sintered ore product can be produced (sintered) to be achieved and maintained.
It has been adopted, in order to reduce an air flowing-through resistance, that a sintering raw material is prepared so as to be of coarse granules by either making primary particles of the sintering raw material larger in size or promoting a granulating nature of the sintering raw material and thereby improving a degree of agglomeration.
In order to promote the granulating nature of the sintering raw material, a binder, such as quick lime, bentonite, cement or cement clinker in powder, has been added to the raw material.
However, in company with recent decrease in output of good quality iron ore, the number of brands of iron ores in use has increased and thereby, a granulating nature of a sintering raw material is largely affected by characteristics of brands. To be detailed, a granulating nature of a sintering raw material has recently decreased with decrease in content of a coarse raw material, use of iron ore of a brand poorer in granulating nature, and increase in content of return sintered ore also poorer in granulating nature. As a result, there has been arisen a problem, since an air flowing-through property of a sintering raw material is degraded, thereby decreasing the strength of a sintered ore product.
In such circumstances, much of a binder has been added to a sintering raw material in order to improve a granulating nature.
However, there has arisen a problem in which a process where much addition of a binder into a sintering raw material leads to increase in production cost of sintered ore. Besides, addition of a large amount of a binder to a sintering raw material makes a composition of an ironmaking raw material fall off a scope allowed as the raw material, thereby influencing operation of a blast furnace adversely.
In addition to the above described problems associated with a binder, an improvement effect on the granulating nature of a sintering raw material has a maximum in regard to an additive amount of a binder and if the binder is added more than the maximum, the granulating nature of the sintering raw material has a chance of degradation.
It is accordingly an object of the present invention to provide a process for producing sintered ore in which a small amount of a water-soluble compound (a sintering aid) is added to iron ore powder and, thereby, which product shows an improved strength and no adverse influence as an ironmaking raw material either without any increase in content of an auxiliary material (CaO) serving as a binder.
The present invention to achieve the above described object is directed to a process for producing sintered ore in which an aqueous solution is added to iron ore powder and an auxiliary raw material and the iron ore powder and the auxiliary raw material are kneaded to form a mixture, and the mixture is granulated to form granules, followed by sintering the granules, wherein the aqueous solution contains a water-soluble compound reacting with the iron ore powder to form a reaction product having a melting point of 1200xc2x0 C. or lower.
Since a compound reacting with iron ore powder to form a reaction product having a melting point of 1200xc2x0 C. or lower and preferably 1150xc2x0 C. or lower is soluble in water, an aqueous solution containing the water-soluble compound makes wet surfaces of particles of the iron ore powder with certainty, so that coating of the surfaces of the particles of the iron ore powder with the compound can be ensured when drying the iron ore powder prior to sintering in a sintering step. As a result, the water-soluble compound and the iron ore powder can efficiently react with each other.
Therefore, the reaction between the water-soluble compound and the iron ore powder can be efficiently performed at a conventional sintering temperature ranging from 1150xc2x0 C. to 1200xc2x0 C. for sintered ore to form a melt, with the result that melt formation through a slag reaction between the iron ore powder and the auxiliary material is accelerated in the presence of the melt from the water-soluble compound and thereby, a sufficient amount of a melt for sintering the iron ore powder is additionally produced, which enables the strength of a sintered ore product to be improved.
Further, as described above, since an aqueous solution containing a water-soluble compound makes wet surfaces of particles of iron ore powder with certainty so that coating of the surfaces of the particles of the iron ore powder with the compound can be ensured to the full extent, therefore the water-soluble compound which the aqueous solution contains can be reduced in quantity, for example to be 1 mass %, and there can be produced a sintered ore product showing no adverse influence on operation in a blast furnace to be otherwise exerted because of the presence of an element constituting the water-soluble compound therein.
Furthermore, an aqueous solution preferably contains a water-soluble compound producing a reaction compound having a melting point ranging from 550 to 900xc2x0 C. through a reaction with iron ore power.
A water-soluble compound which an aqueous solution contains reacts with iron ore powder to form a reaction compound having a melting point ranging 550 to 900xc2x0 C., thereby, a melt can be produced at a temperature lower than a conventional sintering temperature ranging 1150 to 1200xc2x0 C. of sintered ore, and thus produced melt accelerates production of a additional melt through a slag reaction between the iron ore powder and an auxiliary material to a sufficient amount of the melt for sintering the iron ore powder, which enables the strength of a sintered ore product to be improved.
A water-soluble compound of the process of the present invention plays a role of a sintering aid accelerating sintering through a slag reaction between iron ore powder and an auxiliary material.
That is, migration (diffusion) of components of the iron ore powder and the auxiliary material becomes easy with the help of a melt produced by a reaction between the water-soluble compound and the iron ore powder, which accelerates production of an additional melt through a slag reaction between the iron ore powder and the auxiliary material, whereby, it is estimated, sintering of iron ore powder is sufficiently progressed.
Furthermore, it can be expected that a component of a melt produced by a reaction of a water-soluble compound with iron ore powder reduces a formation temperature of the additional melt through a slag reaction between the iron ore powder and the auxiliary material. As a result, more of the melt than in a conventional case of sintered ore production is produced, thereby enabling formation of a wider bond contributable to increased strength of a sintered ore product.
Besides, since as a melting point of the melt decreases as a result of a slag reaction between the iron ore powder and the auxiliary material, a viscosity of the melt also decreases, it can be further expected that the melt is easier to migrate over surfaces of particles of the iron ore powder. As a result, the melt spreads all over the surfaces of particles of the iron ore powder in a uniform distribution, which makes it possible for a network structure of bond contributing to increased strength of a sintered ore product to be formed with spatial uniformity.
As compounds used in a process for producing sintered ore of the present invention, there can be used acmite based compounds such as Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compounds and Na2Oxe2x80x94SiO2 based compounds.
A Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compound can easily react with iron oxides such as Fe2O3, FeO or the like in the iron ore to incorporate the iron oxides into the compound as a solid solution. A solid solubility of the iron oxides has a wide range. Melting points of the Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compounds range from 760xc2x0 C. to close to 1200xc2x0 C. according to a chemical composition thereof and has a wide composition range having a melting point of 900xc2x0 C. or lower.
At this point, by using the Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compounds in a process for producing sintered ore of the present invention, the compounds are molten at a temperature lower than a conventional sintering temperature ranging from 1150 to 1200xc2x0 C. for sintered ore to form a melt, with the result that the melt reacts with iron oxides in the iron ore. Further, the iron oxides are dissolved in the melt of the Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compounds and accelerate further formation of a melt, wherein since a composition range having a melting point of 900xc2x0 C. or lower is wide, formation of the melt is accelerated. With the melt, still further formation of a melt can be accelerated through a slag reaction between the iron ore powder and the auxiliary material, leading to formation of a sufficient amount of the melt for sintering of the iron ore powder, and thereby enabling the strength of a sintered ore product to be improved.
That is, when the Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compounds of compositions having a melting point of 900xc2x0 C. or lower are used in a process for producing sintered ore of the present invention, a melt can be produced at a temperature lower than the above described sintering temperature, or of 900xc2x0 C. or lower, thereby enabling a further increase in strength of a sintered ore product.
In addition to this, the Na2Oxe2x80x94SiO2 based compounds can also be used in a process for producing sintered ore of the present invention. Melting points of the Na2Oxe2x80x94SiO2 based compounds range from about 1020xc2x0 C. to about 1090xc2x0 C. ANa2Oxe2x80x94SiO2 based compound also reacts with iron oxides in the iron ore powder as described above with ease and a melt of a Fe2O3xe2x80x94Na2Oxe2x80x94SiO2 based compound is formed through dissolution of the iron oxides into the Na2Oxe2x80x94SiO2 based compound to form a solid solution. Thereafter, formation of an additional melt through a slag reaction between the iron ore powder and the auxiliary material is accelerated, leading to formation of a sufficient amount of the melt for sintering of the iron ore powder, and thereby enabling the strength of a sintered ore product to be improved.
As water-soluble compounds used in a process for producing sintered ore of the present invention, sodium silicates (Na2Oxe2x80x94SiO2 based compounds) are preferably used. Since sodium silicates are dissolved in water with ease, an aqueous solution of a compound with a desire concentration can be prepared.
As sodium silicates used in the present invention, not only sodium metasilicate (Na2SiO3) but also anhydrous salts such as sodium orthosilicate (Na4SiO4) can be used. Furthermore, various kinds of sodium polysilicates such as Na2Si2O5 and Na2Si4O9, which can be obtained through hydrolysis of the anhydrous salts, can be used.
A process for producing sintered ore of the present invention has an effect to enable a high-strength sintered ore product, which exerts no adverse influence on operation in a blast furnace to be produced.